7.3.1 Soft Start

The boost converter has an adjustable soft start to prevent high inrush current during start-up. To minimize the inrush current during start-up an external capacitor, connected to the SS pin and charged with a constant current, is used to slowly ramp up the internal current limit of the boost converter. When the EN pin is pulled high, the soft-start capacitor (C_SS) is immediately charged to 0.3 V. The capacitor is then charged at a constant current of 10 μA typically until the output of the boost converter V_S has reached its Power Good threshold (roughly 98% of V_S nominal value). During this time, the SS voltage directly controls the peak inductor current, starting with 0 A at V_SS = 0.3 V up to the full current limit at V_SS = 800 mV. The maximum load current is available after the soft start is completed. As the size of the capacitor increases, the ramp of the current limit slows and the soft-start time increases. A 100-nF capacitor is usually sufficient for most of the applications. When the EN pin is pulled low, the soft-start capacitor is discharged to ground.